Simultaneous multi-slice (SMS) imaging using multiband (MB) excitation and parallel imaging is a powerful technique that can reduce magnetic resonance image acquisition time for anatomical, functional, and diffusion weighted magnetic resonance imaging. SMS imaging is achieved by simultaneously exciting several slices in the region of interest (ROI). With the use of multiple receiver coil channels, the sensitivity profiles of the coils are used to disentangle the slices during reconstruction.
Magnetic resonance imaging (MRI) at high magnetic fields, such as seven Tesla (7T), offers increased signal-to-noise ratio (SNR) and enhanced contrast when compared to conventional field strengths. This increased SNR can be parlayed into unprecedented spatial resolution or faster acquisitions for structural, functional, and spectroscopic imaging sequences. Spin echo preparations are often used for diffusion-weighted imaging and may also be useful for functional MRI at high magnetic fields. However, spin echo imaging at 7T and above faces several challenges that must be overcome in order to capitalize on the potential advantages. Three of these issues are: 1) increased inhomogeneity in the applied radiofrequency (RF) field (B1 field) leading to non-uniformities in signal and contrast when using conventional RF pulses; 2) increased RF power deposition in tissue, as measured by the specific absorption rate (SAR), limiting the strength and number of RF pulses in sequences; and 3) changes in characteristic T1 and T2 tissue relaxation parameters that affect sequence timing and image contrast.
One method of creating multi-slice excitations uses RF pulses with slice profiles that are spatially shifted off-center through a sinusoidal modulation of the wave form. The complex summation of these individual component RF pulses, with shifted spatial slice profiles, results in a single composite MB RF pulse. However, the RF power deposition of this type of MB pulse increases with the number of slices. Due to the quadratic increase of power deposition with field strength, the number of slices achievable by the conventional SMS technique at 7T is restricted by transmitter B1 peak power and SAR safety limits. Furthermore, conventional MB composite pulses, particularly 180° MB pulses used in spin echo SMS sequences, are susceptible to slice amplitude and profile attenuation due to the significant B1 field variation at 7T.
An alternative multi-slice pulse design method, the Power Independent of the Number of Slices (PINS) technique, may be used to produce pulses that excite multiple discrete slices simultaneously, without increasing power deposition above that required for a single slice excitation. This is accomplished by interleaving the single-slice RF waveform, modulated with a comb function, with a gradient pulse train. The RF pulse used as the basis for the PINS pulse can be designed to provide the desired bandwidth (BW) and duration.
The PINS technique has been applied to adiabatic VERSE-DANTE pulses to help combat the effect of B1 inhomogeneity. However, adiabatic pulses generate quadratic phase across the selected slice profile and, in a typical spin echo sequence, a linear phase across the slice can only be achieved through the application of a second, identical adiabatic RF pulse. This twice-refocused approach permits the use of adiabatic pulses in a spin echo sequence, but at the cost of increased echo time (TE) and SAR.